In order to prolong the service life of pressurized-water nuclear reactors, consideration is given to undertaking the complete replacement of one or more steam generators of the reactor, after a certain operating time, in the case where a certain number of tubes of the steam generator have suffered damage due to corrosion, to the impacts of particles transported by the primary fluid or to the thermal or mechanical stresses to which these tubes have been subjected in service.
The complete replacement of a steam generator, the water vessel of which comprises nozzles permitting the connection of the primary part of the steam generator to corresponding primary ducts of the nuclear reacator, requires cutting of the primary ducts in the vicinity of their points of connection with the nozzles, and then the precise positioning of the new steam generator and the welding of the nozzles of this new steam generator to the primary duct portions which have been cut away and premachined.
In order to carry out the operations of cutting away and of premachining of the connecting portions of the primary ducts, as well as the subsequent welding operations, it is neccessary to decontaminate the used water vessel of the steam generator and a portion of the primary ducts situated in the vicinity of the corresponding connecting nozzle of the steam generator.
To this end, use is made of an installation permitting the placing in circulation, in the water vessel and in each one of the portions of primary ducts close to the connecting nozzles, of a decontaminating reagent in liquid form which is recycled after decontamination in processing units disposed in the vicinity of the chamber of the steam generator.
To carry out this decontamination by circulation of liquid, it is necessary to isolate the portion of each one of the primary ducts which is situated in the vicinity of the nozzle for connection to the steam generator, by means of an obturating device which can be placed in position, from the interior of the water vessel of the steam generator, before carrying out the decontamination operation.
French Patent 2,602,853 contains a description of an obturator which can be placed within a duct and which consists of an inflatable balloon.
However, such a device exhibits disadvantages in the case of an application within the context of the decontamination of the water vessel of a steam generator.
In fact, it is difficult to construct a balloon the flexible casing of which is able to resist the pressure and temperature conditions experienced during the decontamination, as well as the corrosive action of the decontaminating liquid. The introduction of the balloon into the duct, although this balloon is not subjected to an internal pressure, may present difficulties to the extent that the balloon is liable to undergo a deformation reflected in a blocking in a curved part of the duct.
Moreover, in the case where the balloon has been correctly introduced, placed in position and inflated within the duct, it is liable to undergo, during the decontamination, a deformation under the effect of the pressure of the liquid which may be reflected in a loss of sealing and in the introduction of decontaminating liquid into the primary circuit of the reactor.
Patent Application EP-A-163,087 has likewise proposed an obturating plug exhibiting inflatable sealing elements and a structure for retention in support on the internal surface of the nozzle of the steam generator. The object of the obturator described in this patent application is to isolate the water vessel from the remainder of the primary circuit, which continues to be filled with water. In the case of the decontamination of the water vessel and of a portion of the primary ducts, the primary circuit is empty of water and the water vessel of the steam generator is supplied with decontaminating liquid under pressure: in consequence the device according to Patent Application EP-A-163,087 is not suitable for the case of the decontamination.